Crime Scene Evidence

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Hair, saliva, flakes of skin… even a single eyelash can serve as a calling card wherever we pass.

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However, it is essential to have a set of “suspects” for comparison. A suspect might be a possible criminal whose presence at a crime scene is being investigated, or a presumed father whose biological relationship to a child is being tested. If the comparison between the DNA fragments (genetic markers) from the reference sample and the unknown sample shows a match, it is very likely—though not obvious and certainly not 100 percent certain—that the suspect is “guilty.”

Before these small DNA fragments can be analyzed, especially when the sample is tiny or contains very little DNA (a hair, an eyelash, and so on), they must be amplified. In other words, many copies of these DNA fragments must be produced so that sequencing is possible. This is done using a technique that became widely known during the COVID-19 pandemic in 2020: the polymerase chain reaction, more commonly called PCR.

Only 0.1 percent of our genome makes us different from other humans, but because the genome is extremely long (it contains about 3.2 billion characters), that 0.1 percent includes many differences!

🕵️ Serving Criminology

In 1986, DNA analysis was used for the first time to solve a crime: the murder of two girls in Leicestershire, England. Since then, forensic techniques have improved greatly. Among other advances, DNA amplification using PCR has made it possible to work with very small samples. This has allowed so-called cold cases—cases that had remained unsolved because of a lack of evidence—to be reopened and investigated using modern forensic methods. Some of these cases date back to the 1950s or even earlier. With today’s techniques, it is even possible to predict a suspect’s hair color or certain facial features.

For DNA to be successfully extracted from biological traces—which are often small and fragile—the collection, storage, and transport of samples must be carried out with extreme care. If the forensic examiner is not aware of this, the evidence may be lost, degraded, or contaminated, and any investigation that might have provided information about a crime will be invalidated.

A team of researchers in the United Kingdom developed a technique that makes it possible to predict the last name of the person who left a DNA sample at a crime scene. How is this done? Last names, like the Y chromosome, have historically been passed down from fathers to sons. As a result, people who share the same last name often share a common Y-chromosome lineage.

A team of researchers in the United Kingdom developed a technique that makes it possible to predict the last name of the person who left a DNA sample at a crime scene. How is this done? Last names, like the Y chromosome, have historically been passed down from fathers to sons. As a result, people who share the same last name often share a common Y-chromosome lineage.

Paternity tests are relatively common today. But what if we wanted to know whether a person from the past is related to us? Is that possible?

The limiting factor is ancient DNA. Obtaining DNA in good condition from two living individuals is relatively easy. The situation becomes much more complicated when DNA must be extracted from ancient samples that have degraded over time, as is typical of human remains. And the challenge is even greater when dealing with DNA from prehistoric ancestors. For this reason, specialized techniques must be developed to extract and analyze ancient DNA.

Ancient DNA is usually found in a very deteriorated state in bones and teeth, which makes extraction extremely difficult. If conditions are favorable, some soft tissue may still be preserved, containing more DNA and allowing for easier extraction. In general, the older the sample, the less likely it is that intact DNA will remain. Of course, much depends on the conditions under which the body was preserved.

The analysis of ancient mammoth DNA has allowed scientists to determine what color those animals were.

Mitochondria have their own DNA and multiply inside cells because, millions of years ago, before animal cells existed, mitochondria were free-living bacteria in the environment. Almost all our cells contain mitochondria (red blood cells are an exception). Unlike nuclear DNA, which we inherit equally from our father and our mother, mitochondrial DNA is inherited only from the mother. This is because, at fertilization, the sperm contributes only its head, which contains nuclear DNA, and does not provide mitochondria. All the mitochondria in our bodies are descendants of those originally present in our mother’s egg cell.

Mitochondrial DNA is especially useful for studying ancient samples because there are thousands of copies per cell (one in each mitochondrion). In addition, because it does not include genetic information from the father, it avoids complications related to possible non-biological paternity. This type of direct inheritance—without mixing DNA from the mother and the father, that is, without recombination—means that mitochondrial DNA is passed down in a highly conserved way from mothers to their children. As a result, it is possible to identify remains even when the reference sample and the evidence sample are separated by several generations.

These are some of the questions that can be addressed by analyzing DNA sequences from human populations. The field that studies these issues is population genetics. By analyzing DNA fragments from hundreds of people from different regions of the world, scientists have been able to outline a kind of family tree for humanity.

Early modern human evolution, however, remains an area of active debate and research. Initially, there were two opposing theories. The replacement theory argues that all modern humans descend from an ancestral population of Homo sapiens that appeared in Africa about 200,000 years ago and later spread across all continents, replacing other human species that had evolved from an earlier expansion of Homo erectus.

The alternative theory, also known as the multiregional theory, proposes that after the first expansion of Homo erectus around 1.5 million years ago, its descendants evolved together into Homo sapiens, despite being geographically separated. Normally, when a population becomes isolated from others of the same species, it tends to evolve in ways that make it different. However, the multiregional theory argues that continuous gene flow—that is, ongoing interbreeding between individuals from different populations—kept human populations genetically similar and led to the emergence of a single species. Today, the most widely accepted view is a synthesis of these two ideas. The evidence supports a primarily African origin for modern humans, combined with a small percentage of genetic inheritance from other human species.